Friday, February 15, 2013

More Rockets, More Russia

This video is much longer than your typical 5 minute clip, but I found it entirely worthwhile. It tells the tale of a bunch of Soviet rocket engines that were moth-balled 35 years ago and recently restored to the land of the living. Pretty amazing story. The technical aspects of what makes these engines so wonderful is a little scary when you think about what's involved. And the explanation of how they got around the really big problem they presented is a little feeble. But they do make at least an attempt to explain what's going on, and it's more than I've heard anywhere else, so they deserve credit for that.

The science and technology of rocket engines is a field all it's own. Oh, it follows the same laws of physics as all other mechanical devices, but the requirements are unique. They are not like airplanes, or motor vehicles, or guns, or any other kind of device. Trying to compare them to any other kind of device is hopeless.

For instance, let's look at the rocket engine's fuel pumps. The pumps are driven by a gas turbine engine. On a Russian NK-33 rocket motor this turbine produces 46,000 horsepower. It probably only weighs a couple of hundred pounds. It does have the advantage of breathing pure oxygen, none of this watered down stuff us air breathers use, and it only has to run for ten minutes or so. It's not like we expect it to do this all day. Still and all, 46 thousand horsepower is a real stink load of power. Makes your nitro-methane burning drag racer look like a toy.

Rocket engines don't really make a lot of sense. You have this flimsy funnel shaped nozzle. You light a fire in the combustion chamber, pump a trainload of fuel into the engine in less time than it takes to shower, the fuel burns, creates a massive blowtorch, and this flimsy tin funnel does not disintegrate. We're really walking along the edge of the precipice here. One little screw up, and blooey, it all goes sky high.

Let's put the mind-boggling part of this aside for a minute and talk about a relatively simple mechanical problem.

Launching a large rocket is a bit like balancing a upright broomstick. As long as you are paying attention, and are quick enough, you can keep the broomstick standing up. Let your attention stray for a moment, or delay a corrective motion momentarily, and over she goes. There are a couple of ways to handle this, but in big rockets they use gimbals. The engines are mounted on pivots so they can be swiveled in any direction. They don't need to move very far. If you need to move them more than a few degrees it's probably too late to correct the rocket's direction anyway. But they do need to move.

Now we have big fat fuel lines (coming from the tanks that make up the body of the rocket) connected to the motor, and one of these lines contains liquid oxygen. Now if the motor is going to move, these fuel lines will need to flex, or swivel, or something. The only way I can see to do it is to run the fuel lines through the pivot points and install rotating seals at these locations, but I have never seen a picture or diagram that would confirm this.

Then you've got the actuators, the devices that will actually tilt the motor in response to control inputs. We're going to need some oomph here in order to muscle around this roaring blast furnace, and do it in a timely manner. We're talking on the order of milliseconds here. Where is this oomph coming from? Never seen any explanation of this either.